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Mass inflow and outflow are not equal

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Old   March 29, 2018, 04:17
Default Mass inflow and outflow are not equal
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Teun Verstraaten
Join Date: Mar 2018
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Hello everybody. I am simulating air flow through an air bearing, where there is an inlet where the pressure is set to 5.5bar and an outlet where the pressure is set to 1.0bar. I have these boundary patches

bottomWall {
type wall;
}
topWall {
type wall;
}
outlet {
type outlet;
}
inlet {
type inlet;
}
wall {
type wall;
}
symmetry {
type wall;
}

where I surpressed the nFaces and startFace for brevity. The whole air bearing has some 4-fold symmetry, so I am simulating just one part. Where the parts should meet I put a boundary called symmetry where everything gets a zeroGradient BC.


Now, I want to see how much load the airbearing can support so I add the following to my controlDict

functions {
#includeFunc patchIntegrate(name=topWall,p)
#includeFunc patchIntegrate(name=outlet,phi)
#includeFunc patchIntegrate(name=inlet,phi)
#includeFunc patchIntegrate(name=bottomWall,phi)
#includeFunc patchIntegrate(name=topWall,phi)
#includeFunc patchIntegrate(name=wall,phi)
#includeFunc patchIntegrate(name=symmetry,phi)
}

I keep track of the mass flow through each face as a check to make sure nothing is off, but sure enough, the numbers do not match. The inflow through the inlet is about twice as high as the outflow from the outlet. Also, there is a small mass flow through the symmetry boundary which should not happen. I believe that the problem lies somewhere with my boundary conditions. I add here the p and U boundary conditions that I used, and as an attachment I have the entire simulation folder without the mesh (was too big). If anyone can see what is going wrong I would be very thankful!

pressure:
inlet {
type fixedValue;
value uniform 5.5E+5;
}
outlet {
type fixedValue;
value uniform 1.0E+5;
}
bottomWall {
type zeroGradient;
}
topWall {
type zeroGradient;
}
wall {
type zeroGradient;
}
symmetry {
type zeroGradient;
}

Velocity:
inlet
{
type zeroGradient;
}
outlet {
type zeroGradient;
}
bottomWall {
type noSlip;
}
topWall {
type noSlip;
}
wall {
type noSlip;
}
symmetry {
type zeroGradient;
}
Attached Files
File Type: zip air_bearing.zip (8.1 KB, 3 views)
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